1. Field of the Invention
This invention relates to an optical cable that includes one or more optical fibers surrounded by a silicone waterblocking gel filling material that has excellent low temperature stability.
2. Discussion of Related Art
Optical fibers are very small diameter strands, typically made of glass, which are capable of transmitting an optical signal over great distances, at high speeds, and with relatively low signal loss as compared to standard wire or cable networks. The use of optical fibers in today""s technology has developed into many widespread areas, such as: medicine, aviation, communications, control applications, etc. Many of the areas of use for optical fibers require that the optical fibers be protected from various destructive factors, such as adverse weather, moisture, impact damage, heat, cold, etc.
Optical cables include a core having one or more optical fibers within a sheath system, which -surrounds and protects the fibers. In the cable industry, it is well known that changes in ambient conditions lead to differences in water vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level within the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. Water may also enter the cable because of rodent attacks or mechanical impacts that cause openings in the sheath system. While the presence of water within an optical cable may not immediately impact its performance, the passage of the water along the cable interior to connection points or terminals or associated equipment inside closures, for example, may cause problems, especially in freezing environments and should be prevented.
One way to protect optical fibers is to have a waterblocking gel (also called a water resistant gel) placed around the individual fibers or around groups of fibers. This waterblocking gel minimizes the amount of water or moisture that enters the fiber optic cable and protects the fibers from impact damage during installation, storage, repair, etc. Various materials have been used to fill cable cores to prevent the ingress of water with varying degrees of success.
U.S. Pat. No. 5,574,816 discloses a buffer tube for an optical fiber cable made from a polypropylene-polyethylene copolymer resin (a modified nucleated isotactic polypropylene (i-PP)) having nucleating agents and filler materials disbursed therein. Although materials like i-PP can enable the manufacture of low cost buffer tubes, such material is not compatible with many traditional, low cost polyolefin based waterblocking compounds. The incompatibility is caused by favorable interactions between polyolefin based water blocking gels and the i-PP buffer tube material. As an alternative, polyol based water blocking gels have been developed which offer better compatibility because such materials have a less favorable interaction with the i-PP material. See U.S. Pat. Nos. 5,348,669, 5,433,872 and 5,672,640. However, polyol based gels are generally more compatible with water and therefore allow greater amounts of water to be absorbed into the gels. Additionally, polyol gels may have a greater cost.
U.S. Pat. No. 5,905,833 discloses a filling material which comprises at least two different hydrocarbons as an oil constituent and/or two different fumed silicas as part of a thickening agent. U.S. Pat. No. 5,905,833 alleges that their filling material has enhanced performance at low temperature because of the use of a combination of oils having low resulting pour point and has a high cable drip temperature and very low oil separation.
U.S. Pat. No. 6,160,939 discloses a filling material including one or more synthetic oils and an organic gelling agent (a diblock copolymer).
U.S. Pat. No. 6,085,009 discloses a water blocking gel which is compatible with polyolefin optical fiber cable buffer tubes. The water blocking gel comprises a certain polyolefin oil, a thixotropic agent, and a thermal oxidation stabilizer.
U.S. Pat. No. 5,495,546 discloses a tube containing optical fibers and a flexible, dielectric, water-blocking compound, such as a silicone gel, to prevent migration of water through the tube.
U.S. Pat. No. 6,137,936 discloses filling empty spaces within a buffer tube with a thixotropic polyalphaolefin (PAO) based or a thixotropic silicone oil-based gel filling compound.
UK Patent Application GB 2,046,471 discloses a thixotropic fluid surrounding optical fibers, where the fluid is a colloidal solution or suspension, preferably a silicone oil containing a colloidal suspension of silica, and more preferably incorporating silanol groups in its chemical constitution.
U.S. Pat. No. 4,703,997 discloses an optical fiber cable comprising a water blocking layer, an optical fiber disposed inside the water blocking layer and a water blocking material filling the space between the water blocking layer and the optical fiber, the water blocking material having a specific apparent viscosity and a specific worked penetration.
U.S. Pat. No. 4,374,967 discloses a low-temperature dielectric silicone gel which is the reaction product of a polyorganosiloxane consisting essentially of Me2SiO, MeSiO1.5, Me3SiO0.5 and Me2ViSiO0.5, an organohydrogensiloxane and a platinum catalyst.
U.S. Pat. No. 6,052,502 discloses a water blocking material, preferably a thixotropic material, for example, a silicone gel or a petroleum based material.
U.S. Pat. No. 4,718,747 discloses protective coatings, including layers of silicone rubber or nylon and fillers such as a non-vulcanized thermoplastic compound of ethylene-propylene or polyvinylchloride or a waterblocking filler, such as petroleum jelly or a silicone grease which may include a swelling agent, such as carboxymethylcellulose, etc.
U.S. Pat. No. 4,613,659 discloses a polymethylsiloxane containing silicon-bonded hydrogen consisting of 84 to 96 mole percent dimethylsiloxane units, 2 to 10 mole percent methylsilsesquioxane units, 0.5 to 6 mole percent trimethylsiloxy units, and 0.25 to 3 mole percent monohydrogendimethylsiloxy units, where the total mole percent of trimethylsiloxy units and monohydrogendimethylsiloxy units is at least 1.5.
U.S. Pat. No. 4,464,013 discloses a filling composition containing a styrene based rubber-styrene block copolymer and an oil used to prevent water entry into optical fiber cable.
A cable filling material, especially an optical fiber cable filling material, should meet a variety of requirements. Filling materials for use in optical fiber cables should yield under strains that are experienced when the cable is made or handled. Otherwise, movement of the optical fibers within the cable would be prevented and the fibers would buckle because they contact a surface of the unyielding filling material. Filling materials should also have a relatively low shear modulus. Further, the critical-yield stress may also need to be controlled.
The optical fiber cable filling composition of matter should also not bleed and not drip from the cable core at elevated temperatures. The waterblocking gel should also have good viscosity characteristics at higher temperatures (above 70xc2x0 C.) to avoid having the gel lose its protective characteristics. Another desirable property of the filling material is thermal oxidation resistance.
Water blocking gels for optical fiber cables must also be compatible with the other components of the optical fiber cable, such as buffer tubes, for example, buffer tubes made from polyolefin materials.
It is also desirable that the filling material be substantially free of syneresis, which is to say that it should have uniform consistency over a predetermined temperature range.
Traditionally, three types of waterblocking gels have been used in fiber-optic cables: (1) Gels based on polar oils such as polyols; (2) Gels based on natural or synthetic hydrocarbon oils; and (3) Gels based on silicone oils. In service, the lowest useable temperature of each of these gels is based on the glass transition temperature or on the crystalline melting point of the base oil. Once an oil goes below its glass transition temperature or its crystalline melting point, the oil becomes a solid thereby causing attenuation in the fiber optic elements that can be as high as several decibels per kilometer. U.S. Pat. No. 6,160,939, to Sheu, discloses that known acceptable temperature ranges for optical fiber filling materials are from about xe2x88x9240xc2x0 C. to about 80xc2x0 C. Silicone oils have the advantage of having an extremely low glass transition temperature (about xe2x88x92125xc2x0 C.). However, silicone oils have a crystallization temperature (melting point) of about xe2x88x9245xc2x0 C., thereby effectively limiting the service temperature of cables using silicone gels to temperatures above this.
An object of the present invention is to provide a means to utilize silicone based waterblocking gels in fiber optic cables without experiencing low temperature attenuation caused by crystallization of the base oil at temperatures below about xe2x88x9240xc2x0 C. The use of the silicone gel of the present invention gives the additional advantages of an extremely low glass transition temperature, inherent flame retardancy, good fiber protection from moisture, and compatibility with a wide range of buffer tube materials.
Thus, one embodiment of the present invention is a fiber optic cable that contains at least one optical fiber and a waterblocking silicone gel of polysiloxane based composition that has a crystallization temperature below xe2x88x9240xc2x0 C.
An additional embodiment of the present invention is a fiber optic cable that contains at least one optical fiber and a waterblocking silicone gel that has a crystallization point at a temperature below xe2x88x9240xc2x0 C., and is at least partially composed of poly(alkyl)(aryl)siloxane.
An additional embodiment of the present invention is a fiber optic cable that contains at least one optical fiber and a waterblocking silicone gel that has a crystallization point at a temperature below xe2x88x9240xc2x0 C., and is at least partially composed of halogenated siloxane (mono or polyhalogenated siloxane).